SO2 Adsorption and Transformations on γ-Al2O3 Surfaces: A Density Functional Theory Study

The processes of physical and chemical adsorption of SO2 on clean (100), dehydrated (110), and hydrated (110) surfaces of gamma-Al2O3 have been investigated using periodic density functional theory. In total, 18 stable forms of adsorbed SO2 have been identified on the three types of gamma-Al2O3 surfaces. The computed binding energies of SO2 on these surfaces span the range of 15-70 kcal/mol, which agrees well with the experimental heat of SO2 adsorption determined using thermogravimetric methods. Among these surfaces, SO2 shows a preference to adsorb to the dehydrated surface, and the transformation into surface sulfite was observed. Theoretical vibrational frequencies of these species have been computed, and a good agreement was found with the experimental infrared spectra. It was shown that the characteristic 1060 cm(-1) band on the IR spectra could be attributed, in addition to the proposed sulfate species SO4, to the HSO3 species on the hydrated (110)C surface and the SO3 species on both dehydrated and hydrated (110)C surfaces. The transformations of adsorbed SO2 to SO3/HSO3 were found to be highly exothermic with only moderate kinetic barriers on all the three surfaces.